Text Messages Disappearing

I've noticed that over the past 2 weeks that texts within a conversation are disappearing. Example, my son sent me a picture of his schedule for the week on Sunday. I go back and look on Thursday and the text is gone. I'm not sure if this is occurring with all texts, but I've noticed the occurrence with MMS.

This has just happened to me as well (on a BB Classic) within a thread, with text messages from mid-July to texts during the day just before the disappearing occurred vanishing in an instant. Any insight would be appreciated.